Re: Putting a new yard in
Removing the existing grass with a sod cutter would be the best course of action. By removing the top inch of grass and soil you'll be removing the majority of the weeds currently plaguing your property. Once that is done, you'll want to install or repair the irrigation system. After that, you're going to amend the existing soil with organic materials, compost or manure, then you're going to work the soil with a rototiller, grade for drainage and aesthetics, and finally either seed or sod over the prepared area.
Keep in mind that anything you do to the existing surface will ONLY affect the surface vegetation and do nothing for dormant seed that will be brought forth by stirring the soil with a rototiller. Removal of the top layer will remove most of the issues, but weed seed will always remain. Also, in lieu of a bobcat I recommend a compact tractor with a tiller and bucket. This will move anything you need move and cultivate the soil, all in one machine, whereas a bobcat will only move things, then you're back to the rental yard for another implement to turn and prep the soil which will double or triple your rental costs.
I personally recommend sod, while more expensive than seed, there is none of the negatives that are associated with seed. With sod, you can lay it and within a couple of weeks be out on it and enjoying your lawn and within 6 weeks use it as vigorously as if it'd always been there. With seed you'll get spotty germination, prolonged gentle care, and not get to use it for the duration of the summer. Put the initial cost against the prolonged negatives and sod really is the better choice.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!